The present invention relates to implantable pacemakers, and more specifically to an implantable pacemaker having means for automatically adjusting the stimulation pulse energy so as to efficiently maintain capture.
A pacemaker is an implantable medical device which delivers electrical stimulation pulses to a patient's heart in order to keep the heart beating at a desired rate. The electrical stimulation pulse, when of sufficient energy, depolarizes the cardiac tissue whereat the pulse is applied, causing such tissue to contract. Some pacemakers, and some pacemaker operating modes, provide stimulation pulses at a fixed rate or frequency, such as 70 pulses per minute, thereby maintaining the heart beat at that fixed rate. Other pacemakers, and other pacemaker operating modes, further monitor the heart to determine if the heart is beating on its own (a "natural heart beat"), without being stimulated by a stimulation pulse. If a natural heart beat is detected within a prescribed time period (typically referred to as the "escape interval"), no stimulation pulse is delivered, thereby allowing the heart to beat on its own without consuming the limited power of the pacemaker. (Each stimulation pulse generated by the pacemaker represents an expenditure of energy from the pacemaker's limited power source.) Such pacemakers, and pacemaker modes, are known as "demand pacemakers" because stimulation pulses are provided only as demanded by the heart.
Pacemakers are known that deliver stimulation pulses, and monitor cardiac activity, in the atrium and/or the ventricle of a mammalian heart. Regardless, however, of whether a pacemaker stimulation pulse is applied to the heart atrium or the heart ventricle, the heart must respond to the stimulus provided, or else the energy in the pulse serves no useful purpose, and in fact represents a needless depletion of the limited energy store within the pacemaker. If the stimulation pulse is of sufficient energy to cause depolarization of cardiac tissue, the pulse is said to "capture" the heart. If the stimulation pulse is not of sufficient energy to cause depolarization, the pulse does not capture the heart. "Capture" is thus defined as a cardiac response to a pacemaker stimulation pulse.
Every patient has a threshold which is generally defined as a minimum amount of stimulation energy required to effectuate capture. It is usually desired to achieve capture at the lowest possible energy level setting (in order to conserve energy) yet provide enough of a safety margin so that should the patient's threshold increase, the output of the implanted pacemaker is still sufficient to maintain capture.
Capture is usually determined by means of an electrocardiogram (ECG) measured through ECG electrodes placed on the patient's limbs and/or chest. Some modern implanted pacemakers, electromagnetically coupled to an external programming device, provide an intracardiac ECG without the need for ECG electrodes. When a patient is connected to a typical ECG monitor, or when the patient has one of the newer pacemakers having intracardiac ECG capabilities coupled to an appropriate programming and viewing device, and when the pacemaker is providing stimulation pulses, the physician monitors the output to assess whether each pacing pulse, which is seen as a spike or other marker on the ECG, is followed by a cardiac response. All modern implantable pacemakers are "programmable," meaning that selected operating parameters associated with the pacemaker can be set to a desired value using appropriate noninvasive programming techniques. The amplitude and/or pulse width of the stimulation pulse are typical parameters that can be programmed in this manner. See, e.g., Furman et al., A Practice of Cardiac Pacing, pp 39-55 (Futura Publish Co., 1986). Thus, through experimenting with various settings of the stimulation pulse amplitude and pulse width while viewing the cardiac ECG, the physician is able to determine the approximate "threshold" at which capture occurs. Once this threshold is determined, the physician typically programs the stimulation pulse energy to a level well above the threshold level so as to provide an adequate safety margin.
One type of pacemaker known in the art includes a special calibration mode wherein a series of stimulation pulses of increasing amplitude are generated. Thus, by monitoring the ECG waveform at the same time that this series of increasing amplitude or increasing pulse width stimulation pulses is being applied to the heart, and knowing the approximate amplitude of each stimulation pulse in such sequence, the physician can readily determine the appropriate capture threshold level. See U.S. Pat. No. 3,777,762.
In all of these techniques for determining the capture threshold, and setting the appropriate stimulation energy, it is necessary for the pacemaker patient to physically interact with the physician, which interaction requires time and expense on the part of both the patient and physician. That is, the patient must make the necessary arrangements (and incur the corresponding expenses) of traveling to visit the physician. Likewise, the physician must acquire and set up special monitoring equipment in order to examine the ECG data of the patient. The physician must then manually program the stimulation energy to a level that is well above the determined capture threshold. Usually, due to the imprecise method used to determine the capture threshold, as well as the changes that may occur in the capture threshold over time, a safety factor of at least two is incorporated into the setting of the stimulation energy. Thus, it is quite common for the stimulation energy to be set twice as high as it actually needs to be set. Such a high energy setting depletes the limited energy available within the pacemaker more than is necessary. Further, because the capture threshold may change significantly for a given patient over time and for differing physiological conditions, all of the above steps must be repeated on a regular basis.
What is needed, therefore, is a less bothersome, and less expensive, technique for determining the capture threshold, as well as a more precise manner of setting the stimulation energy so as not to expend more energy than is required. What is also needed is a pacemaker wherein the stimulation energy can be automatically adjusted as required in order to adapt to changing capture thresholds.